John 6:64
But there are some of you that do not believe.” For Jesus knew from the first who those were that did not believe, and who it was that would betray him.
Jesus always knows who they are who do not believe. Here John associates them with Judas who would betray Jesus, suggesting Judas did not believe. Besides that, if Judas actually believed Jesus was the Son of God, would Judas have betrayed him? I don't think so. And Judas was a devil, and Jesus chose him to be his betrayer from the first.
64 But there are some of you who do not believe.” For Jesus knew from the beginning who they were who did not believe, and who would betray Him. 65 And He said, “Therefore I have said to you that no one can come to Me unless it has been granted to him by My Father.”
66 From that time many of His disciples went back and walked with Him no more. 67 Then Jesus said to the twelve, “Do you also want to go away?”68 But Simon Peter answered Him, “Lord, to whom shall we go? You have the words of eternal life. 69 Also we have come to believe and know that You are the Christ, the Son of the living God.” John 6:64-69
Here is the key phrase, that defines those of His disciples who no longer believed, as defined by...who they were who did not believe,
From that time many of His disciples went back and walked with Him no more. Then Jesus said to the twelve, “Do you also want to go away?
Judas was in the group, who the scripture defines this way - “Lord, to whom shall we go? You have the words of eternal life. Also we have come to believe and know that You are the Christ, the Son of the living God.”
THEY who no longer believed went away, no longer following Him as disciples:
THEY who continued to believe, continued to follow Jesus as disciples.
JLB